The present invention relates to the manufacture of drawn sheet glass and provides a process and plant for producing drawn sheet glass. The invention includes sheet glass produced using such a process and/or plant.
There are three classical processes for producing a continuous ribbon of drawn sheet glass, namely the Fourcault, the Colburn (or Libbey-Owens) and the Pittsburgh processes, and these date from the first two decades of this century.
These processes may be divided into two categories depending on how molten glass is fed to the root of the ribbon of glass. In the Colburn and Pittsburgh processes the ribbon is fed by surface currents of molten glass held in a drawing tank. In the Fourcault process the glass is drawn from a slot in a one-piece debiteuse of refractory material which is partially immersed in the molten glass in the drawing tank so that glass enters the slot from the depths of the drawing tank to form the ribbon. In a more recent variation of this process, the debiteuse is replaced by a pair of slowly rotating rollers which define the drawing slot.
The present invention is concerned with a sheet glass manufacturing process wherein molten glass formed in a glass melting tank furnace flows continuously to a drawing tank whence the glass is drawn upwardly in the form of a continuous ribbon from a slot defined by a refractory device partially immersed in molten glass in the drawing tank and into a drawing chamber above the tank.
Such below-surface glass drawing processes are recognised to have certain advantages over surface drawing processes such as the Colburn and Pittsburgh processes. For example, when drawing a glass ribbon having a thickness such as is commonly used for window glass, it is generally regarded that the Fourcault below-surface drawing process is easier to run to produce glass of a satisfactory quality as regards its thickness properties. With careful control, the Fourcault process can be used to produce thinner glass than it is possible to manufacture using the Pittsburgh process, and thin glass can be manufactured more easily than when using the Colburn process. It is also known that when using the classical Fourcault process it is easier to control the thickness of the glass produced. This is especially important when manufacturing thin glass.
There are also disadvantages which are associated with the classical Fourcault process, for example certain optical defects are apt to occur in the drawn sheet glass.
Another disadvantage is in the yield and cost of useful glass. The cost of producing drawn sheet glass may be apportioned as follows: the capital cost of the plant; plant maintenance (repair) costs; labour costs; energy costs and raw materials costs. Energy costs may further be divided into the costs attributable to maintaining the plant at its working temperature and the costs attributable to melting the raw materials to form the molten glass. When a given sheet glass production plant is working the only costs which vary significantly with the rate of production of glass are those of the raw material consumed and of the energy required for melting the raw material. It follows that if the yield of useful glass is reduced, the specific cost of producing that glass will be increased.
The yield of useful glass depends in part on the speed of the drawn ribbon. It is known that in the Fourcault process, the drawing speeds utilised are generally lower than in the Pittsburgh and Colburn processes.
The yield of useful glass also depends on the useful width of the drawn ribbon, that is, that part of the ribbon width which meets the desired quaity criteria. In general, marginal portions of the ribbon will not meet these criteria and in practice the ribbon margins are cut off and returned to the glass melting furnace as cullet. When drawing glass through a given debiteuse by the Fourcault process it is known that the useful ribbon width decreases while the low quality ribbon margins increase in width as the glass is drawn to lower nominal thicknesses. This phenomenon becomes particularly important when manufacturing thin glass with the result that wide marginal portions of the ribbon are of unacceptable quality and are only usable as cullet. It would be possible to avoid a large porportion of this waste by drawing the thinner ribbon from a debiteuse having an appropriately shorter drawing slot, but this would only result in savings in the raw material consumed and in the cost of melting it. The useful glass produced would still be of high specific cost. It is also to be noted that changing a debiteuse is a difficult and time consuming process which in itself results in the loss of potential production of drawn glass.